Put it back together last weekend. Didn't check UCCR but my compression is down to 185 so I'm guessing the calculated 12.25 UCCR is pretty close. Still seems to run pretty good, we'll see how it holds together.

 

"Something I always do when getting 91 octane premium from a pump with a single hose is to put 2-3 gallons of gas in the truck first to flush out the 87 octane that is in the hose"... smart move....!

If you have that much squish clearance and you still get 225 lbs, then you have a very tight bowl. Yup, just looked again at your picture of the head & bowl, you have a 86 cylinder with a 88/89 Bowl. IMO the 88/89 Bowl was not a good design verses the 85/86 or 87 design.

With the thicker head gasket, an 180-190 lbs is a good place to be with your current set-up.

Carlos

 

Checked my gauge last night against the one on my compressor and 1 other gauge that I have, it was right on the money with the other 2 gauges from 0psi to 155 that my compressor pumps out so I'm pretty confident in my gauge.

Also, we checked a buddies bike last night, it has a practically brand new 85 ATC crate motor on it. It came up to 13.01 UCCR, has a stock piston and 1mm gasket. I was a bit surprised by that. His compression was 185 with my gauge.

 

Checked my gauge last night against the one on my compressor and 1 other gauge that I have, it was right on the money with the other 2 gauges from 0psi to 155 that my compressor pumps out so I'm pretty confident in my gauge.

Also, we checked a buddies bike last night, it has a practically brand new 85 ATC crate motor on it. It came up to 13.01 UCCR, has a stock piston and 1mm gasket. I was a bit surprised by that. His compression was 185 with my gauge.

Yeah those ATC's were right on the edge for pump gas 8.0:1 CCR and ~13:1 UCCR but the pump gas was better in the 80's, no ethanol like today. Sounds like your gauge is on the mark, the ATC250R service manual calls out 171-199 psi cylinder pressure and you're smack dab in the middle!

 

even with newer motors setting the squish properly has a big effect. just do some searching on the RB head mods for newer yz250's big increases

 

Squish clearance measured .080" and was at the outer edge of the head. When I change to the oem gasket, that number should increase to .130". Seems like alot of clearance to me.

 

Thanks guys, I'm getting a better feeling now.

Other thing is, I tried to check the squish clearance last night, with the .0098" head gasket, and with .060" solder, there was no squish. I'm going to get some bigger solder and try again tonight. Is it important that it is close or is .060"+ good enough?

I may have to send this head to you acecarlos to get it where I want.

 

12.25:1 is a little edgy for 91 keep good quality fuel in that baby.

 

12.25:1 is a little edgy for 91 keep good quality fuel in that baby.

I'm thinking 91 will be fine for the trails and cruising around riding that I usually do, I'll get some better gas for the dune trips and drag racing.

 

12.26/1...sounds much better and much safer with 91 oct. I did not check your math, just don't have the time. Now check your gauge, I bet it is reading high and your true compression is closer to 195lbs.

Carlos

 

Don't get too hung up on gauge readings a buddy's old KTM 300 kicked about 240 psi and ran fine on pump gas; some YZ250s too search the net a bit. More important is volume above TDC, head design, squish thickness, timing and jetting. I would expect a healthy unported cylinder to kick over 200 psi with a thin head gasket. My ATC does and runs great on 100 pump.

 

Possibly the cylinder was machined to increase compression.

At TDC, does the piston (outer edges) sit flush or below the top deck?

You normally don't get 225lbs going from the stock thicker gasket to the thin .0098" OEM gasket. Stock OEM is 165-175lbs...

Have you checked your compression gauge?

Carlos

 

The piston sits down in the bore a bit, I would say close to .060" but that is just a guess.

I agree that 225 seems high, but it would probably correlate with the 14.99 UCCR wouldn't it?

I have not had the compression gauge checked, I really don't have a way to hook up with snap on anymore. Actually, I could probably check it against the gauge on my compressor since it has the quick coupling on it.

My plan is to put the OEM gasket on it and re-check UCCR, but honestly I'll probably just run with it. It's too much of a hastle to try to get race gas around here.

 

Well I wonder why it has lasted this long then? It really shouldn't be far from stock if I change the gasket back should it? It looks like the only things changed are the bore by .75mm, the wiseco piston and the thin head gasket. The bore makes a small difference, head gasket is a big change but what about the piston? How much does it increase compression over stock piston?

Thanks for all the help by the way.

 

Photo of piston.

And head.

 

Okay uccr is at 14.99 with the .0098 head gasket. If someone wants to check my numbers are 66.75 bore 72 stroke and cc'd 17.5 at tdc, add .5 for the spark plug gives me 18.

Looks like I should be running above 100 octane minimum.

By my crude estimate a stock gasket would put me close to 13.75 and hopefully it would be 91 octane friendly.

 

Okay uccr is at 14.99 with the .0098 head gasket. If someone wants to check my numbers are 66.75 bore 72 stroke and cc'd 17.5 at tdc, add .5 for the spark plug gives me 18.

Looks like I should be running above 100 octane minimum.

By my crude estimate a stock gasket would put me close to 13.75 and hopefully it would be 91 octane friendly.

Actually at 14.99/1...you need a Motor Octane (MO) of 107-108, or a race Fuel like VP C12 or Sunoco Supreme (15/1) don't go by RON (Research Octane Number) as it can be misleading.

At 13.75/1 you still need a race Fuel like VP 110 or Sunoco Standard (14/1)...you will need to be below 12/1 to safely run 91 Octane.

Carlos

 

i used to run a calibration lab and calibrate guages every day some in one year move 10-15lbs we had to check them against a known PSI and Known working guage every 3 months

 

For the record, you did do it right. Throttle WOT, kick until the gauge stops moving anymore. That's the way to do it. Since you used a Snap-On gauge, the fastest way to find out if that gauge is correct is to tell your local Snap-On truck guy to test it - he'll be able to say whether it's right or not, and fix it if it's not right. That service that they have to provide is why the tools from them cost what they do. Make the guy check it, and fix it if it's off.

 

Thanks. Got it apart on the bench. Head gasket was .0098. I'm going to go ahead and do uccr but I got a new gasket coming as well.

 

I used a mac tools set and a Matco compression tester set they were 40 pounds different Both new then I switched around the gauges and they were still not comparable to each other. 1 read 145 and the other read 180ish on my pump gas 330. I can believe theres that much differance. You would think a gauge is a gauge (name brand atleast).

 

Thin CR250R Head gaskets are .0098" or .25MM...if the head/Dome and cylinder have been Decked, than the 225 lbs is very possible. One quick fix is remove the head & thin gasket, and install a new 3 piece metal TRX250R OEM Head gasket (1.6MM thick .063")...much thicker and will reduce compression below 200 lbs and in a safer area for 91 Oct.

Again; only real way to determine what Octane level you need for your cureent or future set-up is to perform a full UCCR., and not by compression alone.

Carlos

 

It kicks hard is why I checked it in the first place. Its a snap-on gauge but I suppose it could be off. My buddy borrowed it right after and got 185 on his stock bike. It has what looks to be the temp gauge sold by bdt on the hose coming out of the head and the highest I have seen it was 190 after idling on a long slow trail. It runs 170 drag racing pass after pass. I don't know the thickness of the gasket but it is visually much thinner than the one on my buddies 86.
I have no idea on piston run in, I know it lasted through a trip to the dunes and me beating on it for 3-4 hours. Haven't done a new plug chop but it looks good after a long run on an old plug. Probably 3-4 mm fuel ring at the base of the insulator.

I guess I'm down to do the uccr.

 

dont run your motor on 91 octane with 225psi cause you will destroy your motor, for 225psi you should be using 110/112 race fuel. if you want to run 91 octane you will have to drop the compression down to 165/175psi.

if you run 91 octane with 225psi your motor will detonate and run very HOT.

carlos and neill will beable to advice you better than me.

kevin

 

Compression gauges can be off and lot's of times they are. If you can double check with a good known gauge...first. Likely your true compression is closer to 175-185 lbs than 225 lbs.

Stock 86 OEM head? How thick is the Head gasket? what does the piston wear or run-in time look like? Plug Chops? Engine temp?

If you truely are pumping out 225 lbs, you need to run 100% Race Fuel...if you really want to know; perform a full UCCR, only waty to really know what you have.

Carlos